A downhole bearing assembly used in earth drilling consists of an outer tubular member, an inner tubular member telescopically received within the outer tubular member, and bearings disposed between the inner tubular member and the outer tubular member to facilitate relative rotation of the outer tubular member and the inner tubular member while accommodating radial and axial loads. Axial loading can be in either of two directions; commonly referred to as compression or tension. Compression loading is the greater of the two in an earth drilling application as the weight of the drill string is brought to bear on the bearing assembly. Tension loading is placed on the bearing assembly whenever the drill bit is lifted off the bottom of the bore hole.
It is the practise in the art to separate the function of the thrust bearings when designing bearing assemblies. One group of bearings take compression loads and a different group of bearings take tension loads. Of course, the load capacity of the bearings taking the compression loads must be much greater than the bearings taking the tension loads. The bearings taking the tension loads, commonly referred to as the "off bottom" bearings have become a weak link in the design of bearing assemblies. Generally, a single off bottom bearing is used. However, should the drill string temporarily become stuck in the bore hole the upwardly jarring force used to dislodge the drill string often exceeds the load capability of the off bottom bearing and damages the bearing assembly. An obvious solution to the problem would be to add sufficient additional off bottom bearings to increase the bearing assemblies off bottom load capacity. This solution is not viewed as practical as bearing assemblies are used for directional drilling, which requires the bearing assemblies to be as short as possible. Adding additional bearings would inevitably increase the length of the tool.